This invention relates to a mobile communications network, and in particular to a system for maintaining frequency synchronization between the basestations of such a network. More generally, the invention relates to controlling the timing of synchronization updates between network nodes.
In a mobile communications network, including multiple basestations, where a mobile communications device may need to be able to establish wireless communications with any of those basestations depending on its location, it is necessary for the basestations to maintain frequency synchronization. That is, when a particular frequency is allocated for transmissions to or from a basestation, it needs to be able to ensure that it is able to receive or transmit on the allocated frequency, with a very high degree of accuracy.
In a conventional mobile communications network, each of the basestations is provided with a highly accurate crystal oscillator, which is able to maintain the required degree of frequency synchronization.
However, in the case of so-called femtocell basestations, which are intended to be located within a customer's home or small office premises and to provide mobile communications services to a relatively small number of mobile communications devices, such highly accurate crystal oscillators are prohibitively expensive.
The IEEE1588 specification describes a technique for achieving synchronization between nodes of a packet network, for example a network using the internet protocol (IP), to provide time synchronization of a slave at the receiver. This protocol, and other similar protocols, make use of time stamping an accurate clock at the source and transmitting this information, in a packet, to the slave at the receiver. If such packets are sent with short time intervals between them, they can maintain the precision of the receiver clock, but this requires large amounts of data to be sent over the packet data network. If such packets are sent with long time intervals between them, the desired precision of the receiver clock may not be maintained. The IEEE1588 specification therefore states that the time interval between sending synchronisation packets is a compromise between the stability and precision of the local clocks and the communication and computation load imposed by transmission of these messages.